Aircraft landing gear typically comprise a strut which may be fixed or retractable, and sprung or un-sprung: sprung landing gear commonly comprising an oleo strut together with torque links. The upper end of the strut is mounted to the main aircraft structure and an axle is mounted at the lower end of the strut, which may carry a wheel arrangement or any other arrangement e.g. skis, or any combination thereof, requiring steering in order to change direction of the aircraft on the ground. The aircraft may be fixed wing, for example military, commercial or an unmanned aerial vehicle (UAV); or rotary wing for example helicopters fitted with wheels.
The strut has a lower end able to rotate with respect to the upper end about a central longitudinal axis, such that the steering device acts to rotate the lower end of the strut, and hence the wheel carrying axle, in order to steer the aircraft. The steering arrangement may include a self-centring device to return the steering to the centred, straight ahead position once the applied steering torque is released. There may also be a lock out or steering disengagement system, for example via a lock out pin or control from the flight deck. Such a landing gear design may be located at the nose, within the aircraft body or be the main landing gear of the aircraft.
Steering systems for aircraft typically have control from the flight deck via a wheel, tiller or joystick, with mechanical, electrical or hydraulic connections transmitting the controller input movement to a steering control unit. The control unit is commonly a hydraulic metering or control valve, which directs hydraulic fluid under pressure to one or more actuators designed with various linkages to rotate the lower end of the strut or slider of the landing gear leg. Actuator designs vary and, as well as hydraulic actuators, electro-hydraulic actuators (EHA) and electro-mechanical actuators (EMA) are commonly used on aircraft.
Known steering systems have fixed actuators acting on a collar connected via a torque link to the wheel axle. When in operation, the extension or retraction of the actuator arm can lead to the actuator swinging to large angles about its pivot and require large areas of clearance in order to avoid clashes with other parts of the landing gear structure. As a result, the steering device can require a large design space and the maximum achievable steering angle can be constrained. In addition, the size of the device and in particular the actuator size has weight implications, and it is desirable for the steering device to be as small and lightweight as possible.